Recently, a water-absorbent resin (or a particulate water-absorbing agent containing an absorbing resin) and a hydrophilic fiber such as pulp are widely used as a main construction raw material of sanitary raw materials, such as disposal diapers, sanitary napkins, incontinence pads, in order to absorb body fluids. The water-absorbent resin is mainly made of (i) cross-linked partially neutralized polyacrylic acid; (ii) a hydrolyzed starch-acrylic acid graft polymer; (iii) a saponified vinyl acetate-acrylic acid ester copolymer; (iv) hydrolyzed acrylonitrile copolymer or cross-linked acrylonitrile copolymer; (v) hydrolyzed acrylamide copolymer or cross-linked acrylamide copolymer; (vi) a cross-linked cationic monomer and the like.
There have conventionally been needs for a water-absorbent resin having the following absorption properties: (i) a high absorption capacity for a aqueous liquid such as a body fluid, (ii) an excellent absorption rate, (iii) excellent liquid-permeability, and (iv) excellent gel strength of a swollen gel, and (v) an excellent suction amount from a wet substrate containing aqueous liquid, (vi) and the like. To meet these demands, a recent particulate water-absorbing agent has a particular physical property.
In view of the indispensable demands for high water absorption capacity and high liquid permeability under pressure, and needs for water-absorbent resin particles significantly narrow particle size distribution, or a water-absorbent resin particles having a high absorption capacity and low water-soluble component, many parameter patents and measurement methods have been applied so as to regulate the various physicalities of water-absorbent resin. The following documents 1 through 39 are example of the patents.
Document 1 discloses a water-absorbent resin superior in gel strength, water-soluble component, and water absorption capacity. Document 2 discloses a water-absorbent resin superior in liquid-permeability without load, absorption rate, and absorption capacity. Documents 3 through 6 disclose techniques of making specific particle size distribution. Further, many other documents suggest a water-absorbent resin superior in absorbency against pressure under different pressures, as well as measurements methods for checking the absorbency against pressure. The following Documents 7 through 16, 38 and 39 suggests water-absorbent resins, some of which are superior solely in absorbency against pressure, and others are superior both in absorbency against pressure and in other physical properties.
Further, Documents 17 and 18 suggest a water-absorbent resin which has excellent damage-resistance of its properties, Document 19 suggests a water-absorbing agent whose dust-size particle amount is specified, and Document 20 suggests a water-absorbent resin causing less coloring. Further, Documents 21 and 22 suggest a water-absorbent resin superior in water absorption amount and gel-stability against an L-ascorbic acid aqueous solution or the like for regulating urine-resistance, and Document 23 suggests a water-absorbent resin superior in air permeability. Furthermore, Document 24 suggests a water-absorbent resin having less residual monomer.
Further, Documents 25 through 32 and some other documents suggest that a water-absorbent resin with a specific physical property is useful for an absorbent article (disposal diaper) with a specific physical property, structure and polymer content. Document 33 etc. suggest a method for surface-cross-linking which is performed by pulverizing at least a part of resin particles.
Further, in recent years, the thickness of a sanitary raw material, such as a disposal diaper, tends to be reduced, making water-absorbent resin content higher, which increases a demand for relieving odor from the water-absorbent resin. To relieve the odor, there has been suggested use of various deodorant agents and/or antibacterial agent together with a water-absorbent resin, so as to give a deodorant property to the water-absorbent resin. Examples of this product includes a water-absorbent resin composition made of a water-absorbent resin and an extract from leaves of a theaceous plant (for example, Document 34), or a water-absorbent resin composition made of a water-absorbent resin having a specific property and an extract from an aiculilignosa (for example, Document 35). Further, some recent technologies (e.g. Document 36) focus on acetic acid or propionic acid, which are impurities in the acrylic acid, as a cause of odor of water-absorbent resin. Further, other technology relating to a residual monomer (e.g. Document 37) focuses on β-hydroxypropionic acid, which is an impurity in the acrylate.
As described, there have been many technologies for meeting various demands regarding property of water-absorbent resin. However, those conventional water-absorbent resins can no longer ensure their effects in the recent absorbent structure (such as disposal diaper) which uses a greater amount of water-absorbent resin, that is, has a higher content (higher weight ratio) of water-absorbent resin. Moreover, as the usage amount (content) of water-absorbent resin for an absorbent structure, such as disposal diaper, is increasing, the deodorant property has become an important factor, but trials of various deodorant agents, which are used to give the deodorant property direct onto the absorbing agent, have not made satisfactory results of obtaining sufficient deodorant property.
Further, an absorbent article, such as disposal diaper, has been having defects of coarse feeling of absorbent particles, and particle segregation which makes particle size uneven. In view of these problems, there is another demand for water-absorbent resin particles having narrower particle size distribution.
However, any of conventional water-absorbent resins have not satisfied all of these demands in actual use: reduced odor, excellent absorption capacity, narrower particle size distribution and less particle segregation.
In view of this, the present invention provides a water-absorbent resin satisfying all of the demands in actual use: reduced odor, excellent absorption capacity, narrower particle size distribution and less particle segregation. With this water-absorbent resin, the present invention also provides an absorbing agent which can be used as a comfortable absorbent article ensuring high physical property and safety and has a composition not causing odor after swelling.                [Document 1] U.S. re-issued Pat. No. Re32649 Specification        [Document 2] UK Patent No. 2267094 B Specification        [Document 3] U.S. Pat. No. 5,051,259 Specification        [Document 4] U.S. Pat. No. 5,419,956 Specification        [Document 5] U.S. Pat. No. 6,087,002 Specification        [Document 6] EP Patent No. 0629441 Specification        [Document 7] EP Patent No. 0707603 Specification        [Document 8] EP Patent No. 0712659 Specification        [Document 9] EP Patent No. 1029886 Specification        [Document 10] U.S. Pat. No. 5,462,972 Specification        [Document 11] U.S. Pat. No. 5,453,323 Specification        [Document 12] U.S. Pat. No. 5,797,893 Specification        [Document 13] U.S. Pat. No. 6,127,454 Specification        [Document 14] U.S. Pat. No. 6,184,433 Specification        [Document 15] U.S. Pat. No. 6,297,335 Specification        [Document 16] U.S. re-issued Pat. No. Re37021 Specification        [Document 17] U.S. Pat. No. 5,140,076 Specification        [Document 18] U.S. Pat. No. 6,414,214B1 Specification        [Document 19] U.S. Pat. No. 5,994,440 Specification        [Document 20] U.S. Pat. No. 6,444,744 Specification        [Document 21] U.S. Pat. No. 6,194,531 Specification        [Document 22] EP Patent No. 0940148 Specification        [Document 23] EP Patent No. 1153656 Specification        [Document 24] EP Patent No. 0605215 Specification        [Document 25] U.S. Pat. No. 5,147,343 Specification        [Document 26] U.S. Pat. No. 5,149,335 Specification        [Document 27] EP Patent No. 0532002 Specification        [Document 28] U.S. Pat. No. 5,601,452 Specification        [Document 29] U.S. Pat. No. 5,562,646 Specification        [Document 30] U.S. Pat. No. 5,669,894 Specification        [Document 31] U.S. Pat. No. 6,150,582 Specification        [Document 32] International Publication No. 02/053198 Pamphlet        [Document 33] EP Patent No. 0937739 Specification        [Document 34] Japanese unexamined patent publication Tokukaisho 60-158861        [Document 35] Japanese unexamined patent publication Tokukaihei 11-241030        [Document 36] International Publication No. 03/095510 Pamphlet        [Document 37] U.S. Pat. No. 6,388,000 Specification        [Document 38] U.S. Pat. No. 5,409,771 Specification        [Document 39] UK Patent No. 235307 Specification        